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The interest for microorganisms inhabiting the hypolimnion and for their role in biogeochemical cycles of lakes is considerable, but knowledge is far from complete. The presence of chemolithoautotrophic Bacteria and mesophilic Archaea (e.g., Thaumarchaeota) assimilating inorganic carbon in the deep hypolimnion of lakes has been ascertained. We measured, for the first time at 350 m in Lake Maggiore (Northern Italy), the prokaryotic in situ dark [14C]HCO3 incorporation with a new custom-made apparatus, which takes samples and adds tracers in situ. Thereby stress factors affecting prokaryotes during sample recovery from the depth were avoided. We tested the new instrument at different depths and conditions, performing parallel conventional on board incubations. We found that dark [14C]HCO3 incorporations had lower standard deviation in in situ incubations with respect to the on board ones, but their means were not statistically different. At 350 m we estimated an uptake of 187.7±15 μg C m–3 d–1, which is in line with the published uptake rates in aquatic systems. By inhibiting the bacterial metabolism, we found that Archaea were responsible for 28% of the total CO2 uptake. At the same depth, Thaumarchaeota, on average, constituted 11% of total DAPI counts. Dark [14C]HCO3 incorporation integrated along the aphotic water column was 65.8±5.2 mg C m–2 d–1 which corresponds to 87% of picophytoplanktonic autotrophic fixation in the euphotic layer. This study provides the first evidence of Bacteria and Archaea dark CO2 fixation in the deep hypolimnion of a subalpine lake and indicates a potentially significant prokaryotic CO2 sink.
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